The present invention relates to pilot valves and, more particularly, to a guard for a pilot valve.
Automatic safety systems employing guards for pilot valves, which are also called pilot guards, are often used to control burners within fired equipment, such as to heat crude oil that has been collected in vessels in order to facilitate the separation of water droplets from the crude, which may be deployed in remote locations and be unattended (the word xe2x80x9ccontrolxe2x80x9d as used herein simply means on-off accessibility to the fuel supply, i.e., access to the fuel supply is permitted in the xe2x80x9conxe2x80x9d position and is precluded in the xe2x80x9coffxe2x80x9d position; whether fuel is actually directed to the main burner is determined by another valve, responsive to its own thermostat, interposed between this burner and the pilot guard). Such systems for both the pilot and main burner are required to avoid accumulation within the fired equipment of raw fuel discharged by unlit burners in volumes sufficient to be an explosive hazard. Because the collection vessels may be remotely located, a source of electrical power is often unavailable, or if available, is not reliable. To avoid reliance on electrical power in a control means, prior art pilot guards have utilized materials, such as mercury, which expand greatly when heated. Such arrangements are not desirable because the materials are often toxic, are susceptible to leakage, and since they have a relatively large mass from which heat must be dissipated after the removal of heat, do not react rapidly to failure of the flame being sensed. Many of these prior art devices that did use a thermocouple provided no means for emergency shutdown or means for testing the operation of the safety system.
U.S. Pat. No. 6,065,484 (xe2x80x9c484xe2x80x9d) discloses a burner and pilot guard safety and control system that provides a pilot guard having a stop shuttle normally biased to a seated position to completely block communication with a source of natural gas under pressure and a reset shuttle movable to a reset or start up position in which it unseats the stop shuttle while simultaneously permitting communication of the pilot burner with the gas source and blocking communications with the main burner. A reset latch is arranged to hold the reset shuttle in its reset position until released. A thermocouple capable of producing a voltage output proportional to its temperature is heated by the flame of the pilot burner and is connected to an electromagnet. The electromagnet, when fully energized, holds the stop shuttle in its unseated position. When the reset latch is released, the reset shuttle is then moved by the force of the gas pressure to an operational position in which both the pilot and main burners are in communication with the gas source. A momentary contact switch is arranged, when depressed to its closed position, to short circuit the thermocouple. When the thermocouple is short circuited, the holding force of the electromagnet immediately deteriorates and the stop shuttle is instantly biased to its seated position blocking all communication with the gas source.
The pilot guard that is disclosed in 484 works well in many applications. However, in some applications, the heat generated by the pilot flame is not adequate to energize the electromagnet sufficiently to allow it to hold the pilot guard assembly open after the pilot flame is lit. Consequently, in these applications, both the pilot guard assembly and the main burner valve assembly within the pilot guard will never xe2x80x9clatch inxe2x80x9d and will shut down upon release of the reset shuttle in the event of inadequate heat generated by the pilot flame. This condition of insufficient pilot flame heat could have several causes, including low BTU gas, excessive amounts of secondary air through the fire tube, low pilot pressure, and improper thermocouple alignment. Additionally, a pilot flame that is too hot could increase the time needed to de-energize the electromagnet and consequently shut off the pilot and burner gas upon occurrence of a flame-out to dangerous levels.
Therefore, there is a need for a pilot guard that overcomes the deficiencies of the prior art in handling the problems posed by the variable levels of heat produced by pilot flames.
The present invention provides a pilot guard that is safer and more adaptable than prior art guards, and that has improved gas supply shut-off times in the event of a loss of pilot flame. The present invention can be adjusted in the field to accommodate a variety of levels of heat produced by pilot flames. In the preferred embodiment of the present invention described below, the adjustment is provided by a potentiometer.